P12.4 Ultraviolet Waves, X-rays, and Gamma Rays

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: the short-wavelength end of the spectrum: uses, ionisation and dangers. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how ultraviolet differs from visible light and how fluorescent substances use it
• Describe how X-rays and gamma rays are alike and how they differ
• Describe uses of ultraviolet, X-rays and gamma rays and (Higher tier) explain why each is suitable
• Explain what ionising radiation is and why ultraviolet, X-rays and gamma rays are dangerous
• Describe how people are protected from these radiations

Key words

• Ultraviolet (UV): EM waves with wavelengths a little shorter than violet light (about 10–400 nm). Invisible to the eye.
• Fluorescence: When a substance absorbs ultraviolet and then emits visible light, so it glows.
• X-rays: Very short-wavelength EM waves made when fast-moving electrons are suddenly stopped by a metal target.
• Gamma rays: Very short-wavelength EM waves given out by unstable nuclei of radioactive substances.
• Ionisation: Knocking electrons out of atoms, which leaves them as charged particles called ions.
• Mutation: A change in a gene (in DNA). It can make a cell grow out of control and cause cancer.

Ultraviolet waves

Ultraviolet lies between violet light and X-rays. Its wavelength is shorter than visible light, so its frequency is higher and it carries more energy. Your eyes cannot see it.

Some chemicals are fluorescent: they absorb ultraviolet and then emit visible light. Uses:

• Security marking – an invisible ink mark on a bike or phone glows under a UV lamp; banknotes have hidden patterns that only appear in UV; police can spray a traceable fluorescent liquid on intruders that later glows under UV.
• Energy-efficient (fluorescent) lamps – the gas inside makes UV, and a coating on the inside of the tube absorbs it and glows with visible light.
• Sun tanning and sunbeds – UV makes the skin produce a darker pigment.
• Killing microbes – strong UV can sterilise water and surfaces.

Hazard: Too much UV from the Sun or a sunbed causes sunburn, makes the skin age prematurely and increases the risk of skin cancer. It can also damage the eyes (cataracts, even blindness). Protection: high-factor sun cream, UV-blocking sunglasses, hats and shade, limited time on sunbeds with special goggles. The ozone layer high in the atmosphere absorbs much of the Sun’s UV.

X-rays and gamma rays

• Where in the spectrum; X-rays short-wavelength end; Gamma rays short-wavelength end (usually the shortest)
• How they are made; X-rays fast-moving electrons are stopped by a metal target in an X-ray tube; Gamma rays changes in the nucleus of an unstable atom: the nuclear energy store decreases
• Typical wavelength; X-rays about 10⁻⁸ m to 10⁻¹¹ m; Gamma rays about 10⁻¹¹ m and shorter
• Energy; X-rays high; Gamma rays usually even higher
• Penetration; X-rays pass through soft tissue; absorbed by bone, dense metals and lead; Gamma rays more penetrating; need thick lead or concrete to reduce them
• Ionising?; X-rays yes; Gamma rays yes

Because denser materials absorb more X-rays, X-rays can make shadow pictures: of broken bones (P12.5), of luggage at airport security, and of hidden cracks inside metal parts such as welds – a crack absorbs less, so more X-rays get through there.

Uses of gamma rays

• Sterilising food – gamma rays kill bacteria that cause decay and food poisoning. The food does not become radioactive.
• Sterilising medical equipment – syringes and instruments are sealed in plastic first, then irradiated. Gamma rays pass through the wrapper and kill any microbes; the seal keeps new ones out.
• Treating cancer – narrow beams of gamma rays from a source such as cobalt-60 are aimed at a tumour from several directions.

Worked example: killing bacteria with gamma rays

A pack of chicken contains 4.0 × 10⁶ bacteria. Irradiation kills 99% of them. How many survive?
survivors = 1% of 4.0 × 10⁶ = 0.01 × 4.0 × 10⁶

Answer: 4.0 × 10⁴ bacteria

Worked example: wavelength of gamma rays

Gamma rays from a source have a frequency of 3.0 × 10²⁰ Hz. Calculate their wavelength.
λ = v ÷ f = 3.0 × 10⁸ ÷ 3.0 × 10²⁰ = 1.0 × 10⁻¹² m

Answer: 1.0 × 10⁻¹² m (smaller than an atom)

Ionising radiation and safety

X-rays and gamma rays (and the highest-frequency ultraviolet) carry enough energy to knock electrons out of atoms. The atoms become charged ions, so these are called ionising radiations.

• In living cells, ionisation can damage DNA. Low doses can cause mutations that lead to cancer.
• High doses kill cells – this is used deliberately to destroy tumours, but it also causes radiation burns and sickness.
• The harm done depends on the radiation dose, measured in sieverts (Sv). 1000 millisieverts (mSv) = 1 Sv. See P12.5.

People who work with X-rays or radioactive sources reduce their exposure by: standing behind lead or concrete shielding, keeping their distance, keeping the exposure time short and wearing a film badge (dosimeter) that records the radiation received. If the badge shows too much, the worker must stop working with radiation for a while.

Exam tip: Film badges have a plastic case, not metal, so that radiation can reach the film. Small windows of different materials show which type of radiation got through.

Common mistakes

• Saying ultraviolet has a longer wavelength than visible light. It is shorter (and higher energy).
• Thinking irradiated food becomes radioactive. Gamma rays pass through and kill microbes; they leave no radioactive material.
• Writing that X-rays and gamma rays are made the same way. X-rays come from fast electrons being stopped; gamma rays come from unstable nuclei.
• Saying ionisation “adds” electrons. Ionising radiation knocks electrons out of atoms.
• Thinking a film badge protects the wearer. It only records the dose received.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these types of electromagnetic radiation is not ionising? (Choose from: infrared, X-rays, gamma rays, high-frequency ultraviolet.)

Answer: infrared

Working: Infrared has too little energy per wave to ionise atoms. X-rays, gamma rays and high-frequency UV are ionising.

Question 2. A security mark on a laptop glows when a UV lamp shines on it. What does the ink do? (Choose from: it absorbs ultraviolet and emits visible light, it reflects ultraviolet, it absorbs visible light and emits ultraviolet, it emits X-rays.)

Answer: it absorbs ultraviolet and emits visible light

Working: Fluorescent ink absorbs ultraviolet (which we cannot see) and emits visible light, so it glows.

Question 3. True or false? Food that has been treated with gamma rays becomes radioactive.

Answer: False

Working: False. Gamma rays pass through and kill microbes; no radioactive material is left in the food.

Related lessons

• Previous lesson: P12.3 Communications
• Next lesson: P12.5 X-rays in Medicine
• In this chapter: P12.1 The Electromagnetic Spectrum
• In this chapter: P12.2 Light, Infrared, Microwaves, and Radio Waves
• In this chapter: P12 RP Required Practical: Infrared Radiation

Continue in the full course

This lesson is section P12.4 (AGCSC4104) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.